A variety of fastener configurations have been used for anchoring in soft materials, such as plastics, with varying success. Many fasteners are used in thread forming applications. A pilot hole is formed in the anchoring body, and the fastener threads cut into the wall of the pilot hole as the fastener is inserted.
Different materials present different challenges for the design of fasteners to be anchored therein. A feature of many plastics, referred to as “plastic creep”, makes it difficult to permanently anchor in the material. Plastic creep refers to the dimensional changes that occur over time in a plastic body. It is also referred to as cold flow, and the rate of creep or cold flow often increases with increased temperature. As a result of plastic creep, a joint formed by a threaded fastener anchored in a plastic body can loosen over time. In the past, fastener designs have attempted to compensate for the effects caused by plastic creep, with varying success.
A conventional screw thread can be viewed as an inclined ramp in cylindrical form. As a screw is tightened down, the material in which the screw is driven rides up the ramp and is pulled against the screw head. During final tightening, he material is compressed, the screw itself is stretched and clamping load is generated. If the material relaxes overtime, such as occurs in plastic creep, the material has a natural tendency to slide down the inclined ramp of the screw thread. Clamp load is lost, and the joint loosens.
Difficulties also have been experienced in anchoring threaded fasteners in plastic bodies that are subjected to vibration. Vibration can cause the material to move on the inclined ramp of the screw thread, naturally in the direction of loosening. Thus, it can be difficult to retain a prescribed torque tightening of a threaded fastener in a plastic body.
Due to the nature of plastics, and the structures often made therefrom, it is preferred that a fastener drive into the material relatively easily. However, another problem occurs if the “drive-strip window” is small. The drive-strip window is the difference between the torque necessary to drive the screw in the material and maximum torque that can be tolerated on the tightened screw in the material before strip-out occurs in the anchoring material. Large drive-strip windows are desirable to prevent inadvertent strip-out, especially if powered drivers are used to drive the fastener into the material.
Screws are provided in a variety of sizes, both diameter and length. It has been common to provide screws of specific diameters with common thread pitches. However, in doing so it has become known that a screw of one size may work reasonably well in plastic, while a next size larger or smaller having the same general thread design does not work as well. Thus, it has been difficult to design screws of different sizes to work equally well in similar materials.
What is needed in the art is a threaded fastener for plastics and other soft materials which drives into the material easily yet provides a secure joint with improved retention even when experiencing plastic creep or vibration. It is further needed to provide a thread design that can be scaled to provide consistently improved performance with a variety of screw sizes.